Make WebGPU resource creation async and prevent panic during shutdown if WebGPU is enabled.

[kunalmohan]

1. Make WebGPU resource creation async.
2. Remove some unused code in WebGPURequest::RequestAdapter.
3. Prevent panic during shutdown. Since WGPU thread is killed before script, sender and receiver in the script panic at either of the two places-
   a. If a buffer is still alive, script tries to send WebGPURequest::DestroyBuffer to server while dropping the buffer during shutdown.
   

   servo/components/script/dom/gpubuffer.rs

   Lines 118 to 122 in 7170a69

   	impl Drop for GPUBuffer {
   	fn drop(&mut self) {
   	self.Destroy()
   	}
   	}

   servo/components/script/dom/gpubuffer.rs

   Lines 182 to 186 in 7170a69

   	self.channel
   	.0
   	.send(WebGPURequest::DestroyBuffer(self.buffer.0))
   	.unwrap();
   	*self.state.borrow_mut() = GPUBufferState::Destroyed;

   
   b. Receiver in script-thread panics with RecvError as soon as sender on server side is dropped.

   servo/components/script/script_thread.rs

   Lines 1456 to 1457 in 7170a69

   	recv(self.webgpu_port.borrow().as_ref().unwrap_or(&crossbeam_channel::never())) -> msg
   	=> FromWebGPUServer(msg.unwrap()),

r?@kvark

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #25472 (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because ___

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/script_thread.rs, components/script/dom/gpubuffer.rs, components/script/dom/gpudevice.rs
• @KiChjang: components/script/script_thread.rs, components/script/dom/gpubuffer.rs, components/script/dom/gpudevice.rs

[highfive]

Warning

• These commits modify script code, but no tests are modified. Please consider adding a test!

[kvark]

Looks great!

We still have a few senders in message variants that we need to consider fro removal, for example one in CommandEncoderFinish seems unnecessary.

[kunalmohan]

Right. I must have missed that. I'll remove that one here itself.

[jdm]

@bors-servo r=kvark

[bors-servo]

📌 Commit f4d0183 has been approved by kvark

[bors-servo]

⌛ Testing commit f4d0183 with merge 07eb9ab...

[bors-servo]

☀️ Test successful - status-taskcluster
Approved by: kvark
Pushing 07eb9ab to master...